In the past, LCDs have been used in a wide variety of environments, including displays within the cockpit of an aircraft as well as ground-based vehicles. In some of these environments it is desirable to have LCDs with dual mode backlighting. These displays have one mode for use during daylight operation and a second mode for nighttime operation when an observer may be using a night vision imaging system, hereafter ‘NVIS.’ It has been known to use filtered light during the NVIS mode as typical night vision components are sensitive to light within a certain wave length range. Filtering the light allows the LCD to be used simultaneously with NVIS equipment. For example, MIL-STD-3009 (United States Department of Defense Interface Standard Lighting, Aircraft, Night Vision Imaging System (NVIS) Compatible) specifies the small amount of light (at wavelengths longer than ˜650 nm) which may be emitted by an NVIS-compatible display. The disclosure of MIL-STD-3009 is herein incorporated by reference in its entirety. Without other changes, the filtering which helps NVIS performance would degrade the optical performance (mainly color and brightness) of the same LCD when used in daylight operations.
LCDs require a backlight in order to produce an image as these devices do not produce light themselves. Previous devices have used backlights with two sets of light emitting diodes (LEDs): one set for daylight and a second set for nighttime. The set of LEDs which are used during nighttime operations are typically covered by an NVIS filter (sometimes called ‘hot mirror’ filters) which absorb or reflect electromagnetic radiation within a certain wavelength. Some displays which use direct backlighting techniques for the nighttime operations may place a small NVIS filter over each LED. With some displays containing hundreds (or thousands) of LEDs, the manufacturing costs for producing and assembling the many small filters can be very high. Other displays may filter all of the LEDs (even the daytime LEDs) which typically results in reduced color gamut and brightness for the display.
Further, the light uniformity of the backlight is often important and desirable. Because LEDs are point sources of light, it is typically important that their natural illumination is modified to produce a uniform level of illumination across the LCD screen. Light-diffusing (and sometimes scattering) devices have been commonly used for this purpose. However, when used in a direct-lit fashion (as opposed to edge-lit) there must be a space between the LEDs and the diffusing devices (known sometimes as a ‘throw distance’) which forces the entire LCD assembly to become thicker. To further increase the light uniformity of the backlight, a large number of low-power LEDs is typically more desirable than a small number of high-power LEDs. Not only does this increase the light uniformity, but low-power LEDs are typically more efficient than high-power LEDs. Thus, the use of many low-power LEDs can result in power savings as well as smaller amounts of heat generation.
However, as mentioned above, using more LEDs for nighttime operation may increase the manufacturing costs by adding more filters and assembly time to install the filters. This problem is especially troublesome for the nighttime LEDs because the human eye's sensitivity to light variation is much more noticeable at low levels (dim) rather than high levels (bright). Thus, it is very desirable to maximize the uniformity of the light emanating from the nighttime LEDs. Further, it is also desirable to produce the brightest possible daytime LCD with the highest possible contrast ratio while also minimizing the thickness of the LCD assembly and its overall energy consumption.